As truckinusa says, the power for the servos and receiver comes from the BEC, battery eliminator circuit, in the speed controller. There's a tiny voltage regulator inside the speed control that taps a bit of power off the main battery and sends it in through the + and - pins on channel 3.

Here's a neat little piece of information: All the + pins, and all the - pins, inside any receiver are connected together. If you apply power to any set of + and - pins, you will power the entire receiver, no problem!